Clathrate formation in the systems Ba–Ir–Ge and Ba-{Rh, Ir}-Si: Crystal chemistry and phase relations

Phase relations in the {Si,Ge}-rich part of the three systems Ba–Ir–Ge and Ba–{Rh,Ir}–Si were investigated by means of X-ray single crystal (SC) and powder diffraction (XPD), EPMA and DTA measurements. For Ba–Ir–Ge, partial isothermal sections were constructed at 800 °C and 700 °C. The clathrate solid solution extends from binary Ba8Ge43□3 to Ba8Ir0.4Ge43□2.6 with a slightly enlarged lattice parameter a = 2.13402(2) nm ( I a 3 ¯ d , Ba8Ge43□3-type). Two new ternary stoichiometric compounds were found and their crystal structures were determined by X-ray diffraction: τ1-BaIr2Ge7 (Pmna, own type, a = 3.7782(1), b = 0.64099(2), c = 0.64401(2) nm; RF = 0.0331, SC-data) and τ2-Ba3Ir4Ge16 (P4/mmm, Ba3Rh4Ge16-type, a = 0.653948(6), c = 2.22849(3) nm; RF = 0.0388, XPD-Rietveld analysis). othermal sections of the Ba–{Rh,Ir}–Si systems were investigated at 950 °C. Whereas in the Ba–Rh–Si system a clathrate-type I solid solution κI-Ba8RhxSi46−x extends within 2.6 ≤ x ≤ 2.9, the corresponding clathrate-I phase in the Ba–Ir–Si system, κI-Ba8Ir1.8Si44.2 ( P m 3 ¯ n , Na4Si23-type, a = 1.0346(1) nm), is not stable at this temperature, but was clearly observed in the as cast state. alysis of temperature dependent atomic displacement parameters (100–300 K) of guest and framework atoms for κI-Ba8IrxGe46−x−y□y revealed Debye (θD) and Einstein (θE) temperatures of θD = 244 K, θE,11 = 85 K, and θE,22,33 = 64 K proving the rattling nature of the Ba-atoms in the large Ge-based tetrakaidecahedral cages. A similar analysis for κI-Ba8RhxSi46−x yields significantly higher Debye and Einstein temperatures, θD = 508 K, θE,11 = 115 K, θE,22,33 = 99 K, i.e. a tight bonding of Ba-atoms to the smaller Si-based cages. This behavior is further reflected by the pronounced difference in the thermal expansion coefficients: α100–300 K = 13.7 × 10−6 K−1 (κI-Ba8IrxGe46−x−y□y) and α100–300 K = 7.3 × 10−6 K−1 (κI-Ba8RhxSi46−x).